Victron "Energy Unlimited" - Utu

- The house battery will usually be deeply discharged and should really be charged with a high (absorption)
voltage. This is particularly the case when the alternator on the main engine is the only source of power and
runs briefly every day to charge the batteries.
- In contrast, the starter battery and often also the bow thruster battery are practically always fully charged and
do not need any absorption charging.
- Often different battery types are used for starting, for the bow thruster and for house service.
These different batteries all have their own charging recipe.
5.2.3. A wide range of solutions
It would be exaggerating to say that there are as many solutions as boats, but there are certainly many ways to, more
or less, overcome the above-mentioned problems. Several, but certainly not all, will be discussed hereafter:
5.2.3.1 Keeping it simple and low cost: the microprocessor controlled battery combiner
Let the alternator charge the starter battery, and connect the service battery to the starter battery with a battery
combiner (for ex. a Cyrix battery combiner from Victron Energy). When one of the 2 batteries is being charged (the
starter battery by the alternator or the service battery by a battery charger), the Cyrix will sense the increasing voltage
and connect both batteries in parallel. As soon as the voltage decreases the Cyrix will disconnect the batteries from
each other.The advantage is simplicity and cost: the alternator does not have to be modified or replaced.
The drawback is a somewhat longer recharge time of the house battery because bulk charge will stop at
approximately 30 % DoD (or worse in case of important voltage drop in cabling or a low alternator voltage due to high
temperature) and then be followed by float charge. This means that the battery will be cycled between 30 % and
70 % DoD. The solution is to oversize the house battery by 20 % to 50 % and do a 100 % recharge when shore
power is available.
5.2.3.2 Increase alternator voltage
Most alternators with built-in regulators can be modified so as to deliver a higher voltage. Adding a diode in series
with the voltage sense input of the regulator increases output voltage by approx. 0.6 V.
This is a job for the specialist. We will not dwell on it here, but it is a low cost improvement that, together with 5.2.3.1,
will charge batteries quite fast. Severe overcharging is a risk only in case of very intensive motoring every day, and
even that problem can be solved by temporarily switching off the alternator (but never disconnect the main output of
the alternator from the battery with the engine running, because the resulting voltage spike might damage the rectifier
diodes in the alternator).
5.2.3.3 A multi-step regulator with temperature and voltage compensation
When choosing a multi-step regulator (bulk-absorption-float, see chapter 4), I would suggest to go for the best and
choose a model with:
- Voltage sensing. This requires additional voltage sensing wires to measure and regulate voltage directly on
the terminal posts of the house battery or on the DC bus. Voltage-drop in cabling and isolators is then
automatically compensated.
- Temperature compensation. This requires a temperature sensor to be mounted on the house battery.
This solution is often used when an additional high output alternator is fitted.
5.2.3.4 The starter battery.
The solutions as suggested in 5.2.3.2 or 5.2.3.3 will improve charging of the house battery, but what about the starter
battery
Let us assume that when the main engine is running, the batteries are charged in parallel by using battery combining
relays, or a diode or FET isolator. Nearly all of the charging current will then flow to the house battery because this
battery has the greatest capacity, the lowest internal resistance, and is partially or fully discharged. This means that
the voltage drop across the isolator and wiring from alternator to house battery will be higher than from alternator to
starter battery. It might very well be that to achieve an absorption voltage of, say, 14.4 V on the house battery, the
output voltage of the alternator has to increase to 15.4 V (i. e. a voltage drop of 1 V from the alternator to the house
battery).
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